Various atmospheric conditions along the transmission path may affect the strength of a radio signal. In particular, rain fade is one of the most common phenomena to affect satellite signals in a Ka-band satellite communications system. Two of the most common causes of the rain fade are absorption and scattering. When a transmitted radio wave strikes a rain droplet, part of the transmission energy is converted to heat energy and absorbed by the droplet. Moreover, a non-uniform transmission medium due to the raindrops in the atmosphere causes energy to be dispersed from its initial travel direction. These different reactions ultimately have the same effect—they cause any satellite system to lose some of its normal signal level.
To avoid transmission loss due to fade, our copending U.S. patent application Ser. No. 09/875,607 filed on Jun. 6, 2001 and incorporated herewith by reference, suggests using a fallback mode, during which an uplink data rate of a satellite terminal is reduced, for example, from 2 Mbps to 512 Kbps, or from 512 Kbps to 128 Kbps. The maximum fade acceptable for a particular satellite terminal depends on its maximum equivalent isotropically radiated power (EIRP) representing the total effective transmission power. By reducing the uplink data rate of the terminal, the amount of EIRP required to maintain transmission for a given packet loss rate (PLR) is substantially reduced. For example, when the data rate is reduced from 2 Mbps to 512 Kbps, the EIRP required to maintain transmission for a given PLR is reduced by 10 log(2M/512K)=6 dB. Therefore, additional amount of EIRP is available in the fallback mode to prevent transmission loss due to increased fade caused by heavy rains, snow, solar activity or other atmospheric conditions.
The copending U.S. patent application Ser. No. 09/875,607 discloses a feedback-based approach to entering and exiting the fallback mode of operation. This approach utilizes analysis of feedback signals produced by a satellite in response to uplink signals transmitted by a satellite terminal. For each uplink signal transmission, the satellite measures the signal-to-noise ratio (SNR) and signal-to-interference-noise ratio (SINR). This information fed back to the satellite terminal serves as an indicator of excessive fade and is utilized for the fallback mode ingress/egress procedure.
However, the feedback signals are available only when the satellite terminal transmits the uplink signals. If fade increases during a period when the satellite terminal does not transmit the uplink signals, it would be desirable to detect excessive fade before the beginning of the transmission. Otherwise, a substantial delay would occur after the beginning of the transmission before excessive signal fade is detected based on feedback signals. Therefore, it would be desirable to combine a feedback-based fade detection scheme with fade detection based on parameters independent of the uplink signal transmission.